https://www.nasa.gov/mission_pages/launch/countdown101_prt.htmCountdown 101
Follow along with the countdown! The NASA Launch Blog is the only place online to get latest-breaking launch information directly from Kennedy Space Center in Florida. Live countdown coverage will begin about six hours prior to launch and will conclude about 10 minutes after liftoff, when orbit insertion is complete. Coverage includes frequent updates on the countdown status and videos of key events.
T-3 hours and holding
This built-in hold typically lasts two hours.
• Perform inertial measurement unit preflight calibration
• Align Merritt Island Launch Area (MILA) tracking antennas
T-3 hours and counting
• Crew departs for the launch pad and, upon arriving at the pad, begins entry into the orbiter via the White Room
• Complete close-out preparations in the launch pad's White Room
• Check cockpit switch configurations
• Astronauts perform air-to-ground voice checks with Launch Control (Kennedy Space Center) and Mission Control (Johnson Space Center)
• Close the orbiter's crew hatch and check for leaks
• Complete White Room close-out
• Close-out crew retreats to fallback area
T-20 minutes and holding
This built-in hold typically lasts 10 minutes.
• Shuttle Test Director conducts final launch team briefings
• Complete inertial measurement unit preflight alignments
T-20 minutes and counting
• Transition the orbiter's onboard computers to launch configuration
• Start fuel cell thermal conditioning
• Close orbiter cabin vent valves
• Transition backup flight system to launch configuration
T-9 minutes and holding
This is the final built-in hold, and varies in length depending on the mission.
• The Launch Director, Mission Management Team and Shuttle Test Director poll their teams for a go/no go for launch
Image to right: A fish-eye view captures Space Shuttle Endeavour just after liftoff on mission STS-111. Credit: NASA
T-9 minutes and counting
• Start automatic Ground Launch Sequencer
• Retract Orbiter Access Arm (T-7 minutes, 30 seconds)
• Start Auxiliary Power Units (T-5 minutes, 0 seconds)
• Arm Solid Rocket Booster range safety safe and arm devices (T-5 minutes, 0 seconds)
• Start orbiter aerosurface profile test, followed by main engine gimbal profile test (T-3 minutes, 55 seconds)
• Retract Gaseous Oxygen Vent Arm, or "beanie cap"
(T-2 minutes, 55 seconds)
• Crew members close and lock their visors
(T-2 minutes, 0 seconds)
• Orbiter transfers from ground to internal power
(T-50 seconds)
• Ground Launch Sequencer is go for auto sequence start (T-31 seconds)
• Activate launch pad sound suppression system
(T-16 seconds)
• Activate main engine hydrogen burnoff system
(T-10 seconds)
• Main engine start (T-6.6 seconds)
T-0
• Solid Rocket Booster ignition and liftoff!